In recent years, a thin-film solar cell produced using a chalcopyrite compound semiconductor has been put into practical use. The thin-film solar cell produced using a compound semiconductor has a base structure in which layers are formed on a soda-lime glass substrate in an order of a Mo electrode layer, a light-absorbing layer, a buffer layer, and a transparent electrode layer. The Mo electrode layer serves as a positive electrode. The light-absorbing layer is made of a CIGS film. The buffer layer is made of ZnS, CdS, or the like. The transparent electrode layer serves as a negative electrode.
As a method for forming a light-absorbing layer, for example, a method of forming a film by using a multi-component deposition method is known. In a light-absorbing layer obtained by using this method, high-energy conversion efficiency is obtained, but deposition is performed from a point source. Thus, when a film is formed on a substrate having a large area, uniformity of distribution in film thickness is easily degraded. For this reason, a method for forming a light-absorbing layer using a sputtering method has been proposed.
As the method for forming a light-absorbing layer using a sputtering method, firstly, a method (selenidation method) is employed in which an In film is formed by sputtering and by using an In target, a Cu—Ga binary alloy film is formed on the In film through sputtering using a Cu—Ga binary alloy sputtering target, and then a laminate precursor film made of the obtained In film and Cu—Ga binary alloy film is thermally treated in a Se atmosphere, thereby forming a CIGS film.
A technology which will be described below has been proposed on a background of the above technologies. In the technology, a laminate precursor film of the Cu—Ga alloy film and the In film is produced by using a sputtering method which is performed in order of a Cu—Ga alloy layer having a high Ga content, a Cu—Ga alloy layer having a low Ga content, and an In layer from a metal backside electrode layer side. The produced laminate precursor film is thermally treated in a selenium atmosphere and/or a sulfur atmosphere, and thereby the concentration gradient of Ga in the thin film light-absorbing layer is gradually (in stages) changed from an interface layer (buffer layer) side to the metal backside electrode layer side. Thus, it is possible to realize a thin-film solar cell having a high open circuit voltage, and to prevent separation of the thin film light-absorbing layer from other layers. In this case, the Ga content of the Cu—Ga alloy sputtering target being set to be 1 to 40 atom % is proposed.
As such a Cu—Ga alloy sputtering target for forming a Cu—Ga alloy layer, a Cu—Ga alloy sintered body sputtering target which is sintered by performing hot pressing on a Cu—Ga powder mixture produced by a water atomizing device has been proposed (for example, see PTL 1). The Cu—Ga alloy sintered body sputtering target is formed of a single composition. Intensity of peaks other than a main peak in a graph obtained by X-ray diffraction of the Cu—Ga alloy is equal to or 5% less than that of the main peak. An average crystal particle diameter thereof is 5 μm to 30 μm. The oxygen content obtained in the target is 350 ppm to 400 ppm.
In order to improve power generation efficiency of a light-absorbing layer formed from a CIGS film, it is effective that Na be added to the light-absorbing layer through diffusion from an alkaline glass substrate (for example, see NPL 1). However, in a case of a flexible CIGS solar cell in which a polymer film or the like is used as a base instead of the alkaline glass, the CIGS solar cell does not include the alkaline glass substrate, and thus it is inconvenient in that providing a supply source of Na is not possible. In NPL 1, regarding the addition of Na, a method of forming a soda-lime glass between a Mo electrode layer and a substrate is proposed. However, in a case where film formation is performed on the soda-lime glass, producing processes are increased and productivity is degraded. Thus, a technology in which a sodium compound is added to a Cu—In—Ga (referred to as CIG below) precursor film, thereby a supply of Na to the light-absorbing layer is ensured has been proposed (for example, see PTL 2).